Soil softening, Soil erosion - Ground science

Soil softening

Softening of soils is the ability of rocky soils to reduce strength when interacting with water without visible signs of their destruction. The mechanism of softening, like soaking up soils, consists in weakening the structural bonds between particles, grains, crystals as a result of the penetration of water molecules into the spaces between them and reducing the surface energy. Since rocky soils have a high initial strength, they do not lose their full bearing capacity when hydrated and do not soak in water.

To characterize the softness, we use the softening coefficient in water K SO f - the ratio of the strength of the soil on uniaxial compression in water-saturated (R cw ) and in air-dry state (R & lt ;-). The higher the softening factor value (k SO f), which varies from 0 to 1, the less softened is the given primer. According to the degree of softening in the water, rocky soils are subdivided according to Table. 2.2 [34].

The soils under consideration are also classified according to the properties of the skeletal fractions:

soils with a water-resistant skeletal part (softening coefficient above 0.75), which can be represented by fragments of igneous and metamorphic rocks that do not change their strength properties when moistened - granite, basalt, diorite and etc. Most of the igneous igneous and many differences of metamorphic soils are very weakly softened in water: their softening coefficient (k sof ) is within 0.95-1.0;

soils with a non-water-resistant skeletal part (coefficient of softening below 0.75), including fragments of easily eroded, softening rocks when wet. Especially heavily softened soils containing a significant amount of clay minerals (marls, marly limestones, clayey sandstones, clayey shales, etc.), as well as carbonate (limestones, chalk, etc.) and siliceous soils (flasks, diatoms). Easily softened soils with readily soluble cements (sands with gypsum cement, etc.) and water-soluble salts. For example, the coefficient of softening of clayey sandstones does not exceed 0.45, and in some limestones it varies from 0.15 to 0.5.

The softening of soils depends on their mineral composition, the strength of the structural bonds between the elements, the fracture, porosity, etc. To destroy the sample of the solid ground and get a new surface (along the plane of destruction), it is necessary to overcome the cohesive forces that ensure its integrity. In compression, the soil first undergoes volume deformation (elastic and plastic), and then under an effort corresponding to its ultimate strength, is destroyed.

Soil erosion

The erosion of soils is called their ability to decay by the influence of moving water acting on the soil stratum. This property of soils, along with the dynamics of water impact, determines the erosion of the soil massif. Light erosion causes the formation and development of a dense gully network (erosion) and phenomena caused by mechanical suffosion. Depending on the nature and direction of the water impact, they are distinguished (Figure 5.8):

• frontal (wave) erosion of the soil caused by the frontal action of water on the ground;

• surface erosion of soil caused by the action of flowing water along the surface of the soil (tangential),

• Suffocation erosion of soil, caused by removal of soil particles from the massif by a moving water stream.

Fig. 5.8. Soil erosion: a - surface; b - wave; c - suffosion [50}

Wet (frontal) erosion of soils occurs with the frontal action of water on the soil massif. It is widely distributed in the surf zone along the shores of the seas, lakes, reservoirs. In this case, the soil is subjected to periodic shock action of wave energy, possible periodic compression of air in pores, fractured voids and the influence of vacuum phenomena. Wave erosion is the destruction of structural bonds of the soil due to a shock wave and is accompanied by the same accompanying processes as plane erosion, i.e., separation of particles, overcoming their adhesion and their further entrainment from the point of separation.

The intensity of wave erosion of soils depends on the same internal factors as surface erosion, but among the external factors, the wave energy ( in ), the wave approach angle (a ") to eroded surface (plane) of the soil [35].

Surface erosion occurs with the action of flowing waters on slopes (planar erosion), and along permanent watercourses (side and bottom erosion). To characterize the surface erosion of soils, the following indicators are used [50]:

1) the eroding (or critical) water flow velocity (v f/), which is the average flow rate at which the separation begins individual particles and dragging them along the flow;

2) intensity of erosion (/ p ) - the ratio of the average thickness of the eroded layer of soil (DA) at a given erosion rate to the erosion time M), that is,

3) flushing intensity (/ s ), characterizing the loss of weight of the washed earth particles (Δm) per unit time from a unit of flush area and defined by the ratio:

where Δt is the erosion time; S - the area where the particles are washed away.

Pre-admissible, non-smearing average flow rates for homogeneous, non-cohesive soils with less than 0.1 kg/m clay particles having p s = 2.65 g/cm, can be taken from the table. 5.6.

Surface erosion of soils depends on a large number of interacting and interrelated factors, the most important of which are the composition and nature of structural bonds in the soil. The erosion of insoluble soils with rigid crystallization bonds is mainly due to their tectonic disturbance and the effect of weathering processes. The washability of water-soluble soils is determined by the strength of ion-type structural bonds, which dissolve under the action of water, facilitating the removal of the sparingly soluble portion. Dense clays and loams, which do not soak in water, under long-term exposure to flowing water due to their weak lithification, are eroded. The soaked cohesive soils blur the most quickly, and their erosion is largely due to the resistance to soaking. Washability of cohesive soils • depends on the dispersion, chemical-mineral features, porosity, plasticity, soaking, moisture, hardness, adhesion and a number of other characteristics. In general, the higher the strength of structural bonds and their water resistance, the higher the critical eroding rate, and consequently the less the erosion of the soil. The erosion of disconnected coarse-clastic and sandy soils is mainly due to the hydraulic particle size (Table 5.6).

Between the erosion of clay rocks and their soaking, there is a clear relationship: in most cases, quick-eroding soils have a high erosion. Resistance to their erosion increases somewhat with an increase in the content of particles with a diameter of less than 0.05 and 0.001 mm. The erosion of soils depends to a large extent on their structural and texture features. In particular, the resistance of clay soils to erosion will increase with decreasing porosity. In stratified strata, the erosion along the bedding is usually 1.2-1.5 times lower than in the direction perpendicular to it. Resistance to erosion of soils with broken structure is much lower in comparison with undersized soils.

Table 5.6

Permissible feathers washing the flow velocities of homogeneous disconnected soils [106]

Average particle size of the soil, mm

Allowable nondestructive average flow velocities, m/s, with flow depth, m

0.5

1

3

5

0.05

0.52

0.55

0.60

0.62

0.25

0.37

0.39

0.41

0.45

0.50

0.41

0.44

0.50

0.52

1.00

0.51

0.55

0.62

0.65

2.50

0.69

0.75

0.86

0.90

5.00

0.87

0.96

1.10

1.17

25.00

1.46

1.65

1.93

2.12

75.00

2.01

2.35

2.89

3.14

100.00

2.15

2.54

3.14

3.46

150.00

2.35

2.84

3.62

3.96

200.00

2.47

3.03

3.92

4.31

300.00

2.90

3.32

4.40

4.94

Soil suffusion refers to the process of moving small particles of soil through the pores formed by larger particles under the influence of the filtration flow. The mechanism of suffosive erosion, which is sometimes also called the filtration water resistance of soils, consists in hydromechanical action on particles, weakening of structural bonds and removal of individual soil particles along with the filtration flow.

The term mechanical suffusion is introduced in order to distinguish this process from chemical suffusion, when the filtration waters dissolve the chemical compounds of the soil and thereby render soluble inclusions. Mechanical suffusion is manifested in the form of detachment and movement of individual particles, aggregates and whole volumes of soil inside pores or cracks as the most exposed to the suffosion of the soil, and the adjacent soil, the back filter material, backfill, etc. . Chemical suffusion is the leaching by the filtration flow of the mineral base of the soil, which contains soluble substances (gypsum, calcite, halite). The reverse process is called soil colamation - when individual smaller soil particles moving in the pores stop and deposit in some area of ​​the soil mass, clogging the pores. It is also possible chemical colmatation of the soil, when as a result of the chemical interaction of water and soil salts also clog the pores of the soil.

According to [95], any violation of the suffosion stability of the soil should be considered as a violation of the local filtration strength of the base of the structure, the criterion of which is the condition

(5.8)

where i es i is the local pressure gradient in the considered base region, determined by known methods (for example, by modeling the filtering in this area); icr is the local critical pressure gradient, determined from the calculated dependencies or by testing the soil for suffosive stability; y "- Reliability factor by the degree of responsibility of the structure, taken equal to 1.25; 1.20; 1,15 and 1,10 for buildings I, II, III, IV, respectively.

The criterion for ensuring the local filtering strength of rocky soil is the condition (5.8) and the condition

(5.9)

where v. and v wr m - the rate of filtration in the rock massif and the average speed of the water flow

in the gradients of this array; n, is the fractured hollow of the rock massif; v cr j is the speed of water flow in fissures critical for suffusion [95].

Preliminary forecast of the possibility of development of the suffosion process is possible by the indicators of physical properties and composition of the soil. As a rule, the stability of the soil to the internal voluminous suffusion is determined by the degree of filling of the pores of its skeleton with fine earth. According to research by V.S. The source of the phenomenon of suffosive destruction is observed in sands with an inhomogeneity coefficient with and more than 15 ... 20, with values ​​ with and up to 10 ... 15 suffusion damage is observed with gradients of more than 0.8 ... 1.0, while the removal of small particles is observed with smaller gradients. In Table. 5.7 shows the minimum content (according to the VNII VODGPO) of a fine-grained fraction (d <1 mm), at which the filling density of the pores of the skeleton having a loose structure (n = 0, 45). These data should be considered as indicative in the preliminary assessment of the suffosive properties of the soil.

Table 5.7

Characteristics of a resistant to suffusion soil

Small-earth

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Index

plasticity /"

Estimated density, p. g/cm '

Moisture of fine earth. w M

The minimum fine earth content, d.

Sandy loam

0.04

1.60

0.15

0.32

Loam

0.09

1.59

0.165

0.30

Clay

0.25

1.26

0.31

0.24

The optimum humidity, at which sufficient soil compaction is ensured, is determined by the formula

where w e = 0,02-0,03 is the moisture that ensures the wetting of the particles of the skeleton.

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